1. Field of the Invention
The present invention relates to a method and apparatus for feedback controlling the ignition timing and the air-fuel ratio in internal combustion engines to minimize the fuel consumption rate.
2. Description of the Prior Art
Conventionally, unless specific characteristics such as knocking, exhaust emission characteristic, etc. are to be controlled, the ignition timing of an internal combustion engine is controlled in accordance with the operating conditions of the engine in response to the engine speed and intake pipe pressure such that a maximum possible power output of the engine is ensured and also fuel consumption is reduced to a minimum value. However, with previously known methods of this type, it is difficult to always ensure a high degree of control accuracy due to the variations in characteristics among different engines, the variations in environmental conditions or the like. As a result, some degree of loss of power output and fuel consumption are unavoidable.
As regards the air-fuel ratio of mixtures supplied to the engine, under ordinary running conditions, the air-fuel ratio is set to the stoichiometric air-fuel ratio or a leaner air-fuel ratio placing emphasis on the improvement of fuel consumption. During acceleration or while ascending a hill where the accelerator pedal angle is large, the air-fuel ratio is set to the maximum power ratio (about 13:1), and during idling the air-fuel ratio is set to optimize stability, etc. Open loop air-fuel ratio control is performed on conventional carburetors and there is a certain degree of fuel consumption loss due to variations in performance among different engines, the variations over time of engines, variations in performance among the carburetors themselves caused during their manufacture, etc. Closed loop control designed to correct the fuel supply quantity by sensing the direction toward the stoichiometric air-fuel ratio (about 15:1) with an oxygen sensor disposed in the exhaust pipe is put into practical use in a known electronically controlled fuel injection system of the type in which the amount of intake air into the engine is measured by an air flow sensor or the like and the required fuel supply quantity is computed by a computer or the like, thereby injecting fuel into the intake pipe via an electromagnetic valve in accordance with the computed value. The closed loop control for readjusting the amount of air flow through an air bleed by sensing the direction toward the stoichiometric air-fuel ratio with an oxygen sensor is also put into practical use in some known carburetors. However, while closed loop control is effective in preventing variations in the air-fuel ratio, there is still unavoidable fuel consumption loss since the stoichiometric air-fuel ratio does not give the minimum fuel consumption.
A known type of control method for eliminating such a loss and ensuring the minimum fuel consumption is disclosed, for example, in the specification of U.S. Pat. No. 4,026,251. This control method is such that the air bypassing a carburetor is dithered (i.e., the air-fuel ratio is switched between the rich side and the lean side at predetermined intervals) so that the direction of correcting the air-fuel ratio to improve the fuel consumption is determined and the air-fuel ratio is adjusted by an auxiliary air valve bypassing the carburetor. In this method, the engine is operated once at each of two air-fuel ratio levels, namely, relatively rich and lean air-fuel ratios, and then a comparison is made between the rotational speed Ne.sub.R reached in the operation at the rich air-fuel ratio and the rotational speed Ne.sub.L reached in the operation at the lean air-fuel ratio to effect the following control: if Ne.sub.R &gt;Ne.sub.L, then bypassing air is decreased; and if Ne.sub.R &lt;Ne.sub.L, then bypassing air is increased.
However, when a change in the power output is determined in accordance with the rotational speed, since the rotational speed varies in dependence on various factors, this method is not capable of discriminating between a change of rotational speed due to a change of the air-fuel ratio and that due to any external factor (e.g., the accelerator pedal movement, ascending or descending a slope, etc.), so that there is a danger of controlling the air-fuel ratio in a direction opposite to that which improves the fuel consumption and thus deteriorating the fuel consumption.
Now, returning to the discussion to the ignition timing, methods of feedback control of ignition timing to eliminate the previously mentioned loss and ensure the maximum possible engine power are known in the prior art, as disclosed, for example, in the specification of U.S. Pat. No. 3,142,967. In accordance with this method, the engine is operated at each of two different ignition timings which are near the desired ignition timing so that the rotational speed Nr reached in the operation firstly at a retarded ignition timing and the rotational speed Na reached in the operation secondly at an advanced ignition timing are detected and compared as to the relative magnitude thereof. If Nr&lt;Na, the desired ignition timing is advanced further by a predetermined value. If Nr&gt;Na, the desired ignition timing is retarded by the predetermined value. By so readjusting the desired ignition timing, the ignition timing is controlled to the optimum ignition timing which provides the maximum engine torque.
However, when a change in the power output is determined in accordance with the rotational speed, for example, since the rotational speed varies due to various factors, this method is not capable of discriminating between a change of the rotational speed due to the ignition timing and that due to any external factor (e.g., the accelerator pedal movement), so that thus there is a danger such that, during acceleration or deceleration, ascending or descending a slope or the like, the ignition timing is controlled to be readjusted in a direction opposite to that which provides the maximum torque, thus decreasing the rotational speed and deteriorating the power output and the fuel consumption.